1) Field of the Invention
The present invention relates to a suppressor circuit configuration for extending the stable region of operation of a DC driven micro plasma discharge at atmospheric and higher pressures.
2) Description of Related Art
Plasma is a partially or fully ionized gas consisting of various particles, such as electrons, ions, atoms, and molecules. For the quantitative description of plasma, the term of temperature is usually applied. Thermal plasma is in a state where almost all its components are at thermal equilibrium. In nonthermal plasmas (NTPs), temperature (i.e. kinetic energy) is not in thermal equilibrium, and differs substantially between the electrons and the other particles (ions, atoms, and molecules). In this sense an NTP is also referred to as a “nonequilibrium plasma” or a “cold plasma”. Because of the small mass of electrons, they can be easily accelerated under the influence of an electric field. The temperature of electrons typically ranges from 10 000 K to 250 000 K (1-20 eV).
In NTPs, the complex plasma chemistry is driven by electrons. They perform ionization, necessary to sustain the plasma; in addition, they are responsible for atomic/molecular excitation, dissociation and production of “exotic” species. The result is an active gaseous medium that can be safely used without thermal damage to the surrounding. Such exceptional non-equilibrium chemistry is the base of plasma applications in lighting technology, exhaust gas treatment and material processing.
There are several methods to generate non-thermal plasmas, e.g., corona discharge, pulsed corona, microwave, radio frequency (RF) plasma, ionizing irradiation, etc. When charged particles are in minority, heating of neutral molecules is limited. Thus, diffuse plasmas where the fraction of ionized species is below 0.1%, are usually non-thermal. This situation is readily achieved under reduced pressures, in the range of 10 to 1000 Pa. The effect of low pressure is double: in a rarefied gas ionization events are scarce, which keeps the charge density low. Moreover, the frequency of elastic collisions between electrons and atoms/molecules is low, so electrons do not have much chance to convey their energy to the gas. Usually, a discharge in gas is induced electrically, by applying voltage to a set of electrodes. In this case only charged species (electrons and ions) can gain energy from the electric field. The plasmas generated by electric fields are divided into: direct current (DC) discharges, pulsed DC discharges, radio frequency (RF) discharges, and microwave discharges.
Low-pressure plasmas are of great value in fundamental research as well as plasma technology, but they have many serious drawbacks. These plasmas must be contained in massive vacuum reactors, their operation is costly, and the access for observation or sample treatment is limited. Therefore, one of the recent trends focuses on developing new plasma sources, which operate at atmospheric pressure, but retain the properties of low-pressure media.
Non-thermal atmospheric plasmas may be created using one or more of the following principles:
(1) Transient plasmas. The frequency of energy transfer in collisions between electrons and gas is given by v[s−1]=(me/ma)2naδeave where me=ma is electron to atom(molecule) mass ratio, δea is their mutual collision cross-section, na is the atom density and ve is the electron velocity. In atmospheric plasmas n is about 108 collisions/s; for efficient gas heating at least 100-1000 collisions are necessary. Thus, if the plasma duration is shorter than 10−6-10−5 s, gas heating is limited. Of course, for practical purposes such plasma has to be operated in a repetitive mode, e.g., in trains of microsecond pulses with millisecond intervals.
(2) Micro-plasmas. Gas heating occurs in the plasma volume, and the energy is carried away by thermal diffusion/convection to the outside. If the plasma has a small volume and a relatively large surface, gas heating is limited. This situation can be also achieved for a spherical plasma glow.
(3) Dielectric barrier discharges (DBD's). These plasmas are typically created between flat parallel metal plates, which are covered by a thin layer of dielectric or highly resistive material. Usually they are driven by a high frequency electric current (in the kHz range), but it is also possible to obtain a DBD by simple transformation of 50 Hz/220 V network voltage to about 1 kV. The dielectric layer plays an important role in suppressing the current: the cathode/anode layer is charged by incoming positive ions/electrons, which reduces the electric field and hinders charge transport towards the electrode. DBD's have typically low ionization degrees (ion densities of 1019-1020 m−3) and currents in the order of mA. Besides, the electrode plates are quite large (10 cm) and the distance between them usually does not exceed a few millimeters. Thus, DBD has a large surface-to-volume ratio, which promotes diffusion losses and maintains a low gas temperature (at most a few tens of degrees above the ambient). The only serious drawback of a DBD is its limited flexibility. Since the distance between the plates must be kept small, treatment of large and irregular samples is impossible.
In recent decades, non-thermal plasmas have become prominent in surface processing technology. At present, virtually any surface treatment can be performed in a plasma reactor: etching (fabrication of semiconductor elements); deposition of amorphous silicon layers for solar cells; deposition of various thin coatings: hard/protective layers (diamond), nano-structured composite films, cleaning/ashing, tailoring of surface properties: wettability, surface energy, adhesion. The versatility of plasma interactions with various surfaces was the inspiration for a completely new application: plasma-surface treatment in medical care.
Microplasmas are plasmas of small dimensions and may be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that can maintain their state at standard temperatures and pressures are readily available and accessible as they can be easily sustained and manipulated under standard conditions. Therefore, they can be employed for commercial, industrial, and medical applications, giving rise to the evolving field of microplasmas.
Microplasma size ranges from tens to thousands of micrometers and are attractive for commercial applications, e.g., plasma jet, plasma needle, biomedical applications, MEMS technology due to their operational viability and low energy consumption. They are widely used for attaining nonthermal and non-equilibrium discharge at atmospheric and higher pressures due to the fact that their small sizes inhibit the ionization overheating instability through rapid cooling. DC micro plasma discharge operates in a “normal” glow mode at atmospheric and higher pressure. At atmospheric pressure reaction rates are higher and processes can occur more rapidly. For example, the key to having an atmospheric pressure micro-plasma that can be used for plasma enhanced chemical vapor deposition (PECVD) is to provide conditions, which maintain the non-equilibrium state. Non-thermal plasma is required because in PECVD excited and reactive species formed from the precursors are desired. Thermal plasma would result in near complete dissociation of precursors and excessive heating of the substrate.
However, due to their small size, these devices are susceptible to instability from external disturbances. The sources of these disturbances, at many instances, are contributed from the external driving circuits, e.g., the external circuit parameter which triggers self-pulsing oscillations. The oscillation in the negative differential resistance (NDR) region varies from Hz to MHz ranges depending on the parasitic capacitance and discharge current. The effectiveness and reliable operation of DC microplasma devices depends on stable discharge condition and is hindered by the self-driven and sustaining instability resulting from external parameters.
Though parallel plates, pin-plates and micro hollow cathode discharge (MHCD) geometry are the most widely used configurations to obtain a stable discharge for a wide range of current and pressures, the instability in the NDR region is a norm and is unavoidable. However the NDR region in discharge current space is not absolute.
Very few studies have focused on attaining stability in the NDR region of micro plasma operation. The low pressure experiments and subsequent modeling studies in the art demonstrate that self-pulsation of plasma may be suppressed and the region of stable operation can be extended by including a monitoring resistance Rm in series and downstream of the discharge. To attain a stable discharge the monitoring resistance has to be significantly larger than the discharge resistance Rm>Rdischarge. Conditions where an extremely large value of Rm is required for establishing a stable discharge have not been not deemed practical. Ballast resistance being larger than the discharge resistance, i.e. Rballast>Rdischarge, has also been identified/proposed to act as an instability suppressor for low pressure DC discharges operating at low currents because instability of atmospheric pressure microplasma discharges can be suppressed by reducing the parasitic capacitance of the circuit. However, this method has a minimum current boundary due to the practical limits of reducing the parasitic capacitance of the external circuit.
What is needed in the art is a suppressor circuit configuration that can extend the stable region of operation of a DC driven micro plasma discharge—extending the discharge current range of atmospheric and high pressure micro plasma discharge.